Fix LayoutTest transitions/negative-delay.html

The Web Animations implementation always uses keyframe values at offsets of 0
and 1, rather than an interpolated value. This means that at the start of a
transform transition with no underlying transition value, the value used for
the animation is 'none'.

The legacy implementation always uses an interpolated value, so at the start of
a transform transition with no underlying transition value, the value used for
the animation is the identity matrix.

The correct way to represent the computed style of a transform transition is
not speced, but the Web Animations behavior seems reasonable, so this change
updates the expected results accordingly.

Also update the helper script to handle a computed style of 'none' for
transforms.

R=timloh@chromium.org
BUG=303430

Review URL: https://codereview.chromium.org/38703005

git-svn-id: svn://svn.chromium.org/blink/trunk@160510 bbb929c8-8fbe-4397-9dbb-9b2b20218538